The amount of a magnetic flux relative to the armature current is substantially constant in conventional permanent magnet field DC motors having a field system constituted by permanent magnets alone. For this reason, the machine of this kind displays shunt-winding characteristics as the output characteristics thereof and is not capable of generating sufficient torque during starting when a larger current flows therethrough. Another type of permanent magnet field DC machine which is designed in consideration of the magnetomotive force due to the armature reaction is disclosed in Japanese Patent Laid-Open No. 153558/1982 published on Sept. 22, 1982 under the title of "permanent magnet field starter with auxiliary poles". This type of machine has auxiliary poles made of a magnetic material such as soft steel disposed in parallel with permanent magnets in the peripheral direction.
However, according to the conventional methods, the machine cannot generate a sufficiently large starting torque when it starts, and the rotational speed of the machine in the non-loaded state is small because the amount of torque generated by the permanent magnets is significantly large when the machine has no load. For this reason, when the permanent magnet field DC machine is employed as a vehicle starter motor, it acts as a load on an engine of the vehicle after starting the engine while being in the non-loaded state. For the purpose of increasing the starting torque, it is necessary to provide larger areas of the permanent magnets. When rare-earth magnets such as samarium cobalt magnets or neodymium group magnets are employed as the material of the permanent magnets for obtaining same magnetic flux as ferrite magnets and thin magnets, the cost of the field poles thereof is increased, since it is necessary to provide large areas of the permanent magnets.